Unlike a dye, a pigment does not exist in the form of molecules in a medium, but exists as crystal particles in the medium. When the pigment is employed as a colorant in various products, these products are more resistant to degradation and discoloration under light and UV light, in other words, show better light resistance than those making use of the dye as a colorant. Making good use of this property, pigments have found utility as colorants suited for an extremely wide variety of applications such as indoor and outdoor paints, inks, coating formulations, textile printing agents, stationery, color filters, toners, inkjet inks, moldings and otherwise formed products, films, fiber coloring, and so on.
Nonetheless, even such pigments include those inferior in light resistance as will be mentioned below. Pigments include inorganic pigments formed of inorganic compounds and organic pigments formed of organic pigments. In general, inorganic pigments tend to have better light resistance compared with organic pigments, but even such inorganic pigments include those inferior in light resistance. Taking titanium oxide as a pigment, for example, the anatase type and rutile type are known depending on a difference in crystal form. The anatase type is generally inferior in light resistance. Even when organic pigments have crystallized and exist as particles, deterioration unavoidably occurs under UV light as they are organic compounds. As described above, the organic pigments, therefore, tend to be inferior in light resistance compared with inorganic pigments. Among organic pigments, azo pigments are low in light resistance because the azo groups in their structures are prone to degradation under UV light. Even among copper phthalocyanine pigments having high light resistance, ε-copper phthalocyanines are known to be lower in light resistance than those of other crystalline forms.
Even when a pigment exists as crystals, that is, as particles, it may have inferior light resistance depending on its particle size. For example, the light resistance of a pigment tends to become inferior as it is microparticulated. Upon application of a pigment to an inkjet ink, for example, further ultrafine grinding is required from the specialty of the recording method that the ink is ejected from a narrow nozzle. Especially with a yellow azo pigment, its microparticulation results in inferior light resistance or leads to a reduction in the available color range. It is, therefore, the current circumstance that the particle size of a pigment to be used is determined with a compromise. In the case of a pigment for a color filter, on the other hand, it is essential to microparticulate the pigment to increase its contrast and transparency. Here again, there is the problem that its light resistance is lowered because its particle size is made smaller. With a view to overcoming this problem, a measure has been taken such that a protective layer such as an UV-absorbing film is introduced to protect an ultrafinely ground pigment from UV light as much as possible. As described above, however, the light resistance of a pigment depends on its molecular structure, crystalline structure and particle size, and therefore, is determined by its physical properties. Therefore, a limitation may be imposed on the use of the pigment from the standpoint of light resistance. Depending on the application, a desired pigment may not be used.
For providing a pigment with improved light resistance, it is a current practice to change the structure of the pigment or to increase the molecular weight of the pigment, or in the case of an azo pigment, to form a diazo or higher azo pigment having two or more azo groups so that the molecular weight is increased. It has also been proposed to control the particle size or crystalline form of the pigment, or especially to provide the pigment with high crystallizability (see, for example, Patent Document 1).
It has also been proposed to add a light resistance improving additive such as an ultraviolet absorber or light stabilizer when a pigment is added as a colorant upon manufacture of a product (see, for example, Patent Document 2). In addition, it has also been proposed to reduce discoloration of a pigment in a product, in which the pigment has been added, by forming the product into a multilayer structure or overcoated structure with a film, coating formulation or the like in which an ultraviolet absorber has been added, or by applying an UV-absorbing film or coating to a light source to prevent the emission of UV light or to cut out UV light (see, for example, Patent Document 3).